1. Field of the Invention
The present invention relates to a nonradiative dielectric waveguide suitable for use in a transmission line, in an integrated circuit implemented in millimeter wave band equipment.
2. Description of the Related Art
FIGS. 19(A), 19(B), 19(C) and 19(D) are sectional views illustrating the construction of four types of conventional nonradiative dielectric waveguides (NRD guides). FIG. 19(A) shows a normal-type nonradiative dielectric line in which a dielectric strip 100 is provided between conductive plates 101 and 102 which are placed in parallel to each other. FIG. 19(B) shows a grooved-type nonradiative dielectric line in which a groove is formed in each of the conductive plates 101 and 102, with the dielectric strip 100 being fitted into the groove. FIG. 19(C) shows an insulation-type nonradiative dielectric line in which the dielectric strip 100 is provided between conductive plates 105 and 106 via dielectric layers 103 and 104 having a low dielectric constant. FIG. 19(D) shows a winged-type nonradiative dielectric line in which dielectrics 107 and 108 are formed along the plane portion of conductive plates 109 and 110, each of the dielectrics 107 and 108 having a projecting winged portion, and the winged portions are made to contact each other.
In the waveguides, nonradiative dielectric waveguides have opposing conductive plates, and a dielectric strip inserted between the plates. Dielectric layers may also be provided on the surfaces of the conductive plates facing a path formed by the plates.
Electromagnetic wave having a polarization plane parallel to the surface of the conductive plates propagates in the dielectric strip, which is referred to as the "propagating region", while the propagation of such wave is cut-off in the other region, the "cut-off region", between the plates.
In such a nonradiative dielectric waveguide, transmission loss is reduced by making the spacing between the conductors less than a half of the wavelength of the propagating electromagnetic wave, thus suppressing the radiant wave in a bent portion or a non-continuous portion.
However, in the normal-type nonradiative dielectric line shown in FIG. 19(A), highly accurate positioning of dielectric strips to form an electromagnetic wave path is relatively difficult, and such structure is susceptible to vibrations and impact since no mechanism for positioning the dielectric is provided in the conductor planes.
The grooved-type nonradiative dielectric line shown in FIG. 19(B) excels in positioning and the mechanical strength of the wave guide. However, there are problems, for example, current flow concentrated in the corner portions of the groove causes large loss of transmission, and a conductive plate having grooves is disadvantageous in view of the cost of mass production. Furthermore, if a dielectric strip having a high dielectric constant, er, greater than about 5 is used, a small gap between the strip and conductive plate may cause unpredictable changes in the characteristics of the waveguide.
In the insulation-type nonradiative dielectric line of FIG. 19(C), since a dielectric layer of a low dielectric constant is provided between a dielectric strip of a high dielectric constant and a conductive plate, even if the nonradiative dielectric waveguide is reduced in size by using a dielectric material of a high dielectric constant, the problem of the narrowing of the signal operating region due to the occurrence of a high-order mode does not occur. Further, variations of characteristics due to the gap between the strip and the conductive plate are eliminated. However, the insulation-type nonradiative dielectric line has the same drawbacks as those of the normal-type nonradiative dielectric line in the positioning and the mechanical strength of the dielectric strip.
In the winged-type nonradiative dielectric line shown in FIG. 19(D), the above-described problems are eliminated. However, the higher the dielectric constant of the material used, or the higher the frequency used, the thinner the wing portion must be made. Thus, it becomes difficult to produce a dielectric layer having a small winged portion even if an injection molding technology is employed.